1. Field of the Invention
The present invention relates to a passive radar receiver receiving a radio signal comprising frames of symbols each emitted on orthogonal coded carriers.
2. Background Art
In the radar field, it is generally difficult, if not impossible, to achieve the theoretically achievable detection performance limits for a mobile target. This is because detection performances are conditioned by the signal to thermal noise ratio at the output of a tuned filter in the radar receiver and in practise is generally limited not by thermal noise but by clutter at the output of the tuned filter. In the present context the term clutter is to be interpreted in the wide sense of all paths with zero Doppler effect. For example, for a bistatic radar with distant emitter and receiver, the clutter designates all of the following paths: the direct path from the emitter and each path received following reflection by a fixed obstacle.
Various methods of rejecting these unwanted signals are known in the art but have nonnegligible drawbacks. For example, adaptive rejection methods based on using a covariance matrix of the signals received by an array of sensors have the following limitations:
They eliminate only a limited number of decorrelated interference signals, characterized by their direction. Consequently, these methods are not optimized in the context of the fight against clutter when the latter is rich in multiple paths with different time-delays.                They lead to the creation of blind axes, associated with the rejected signals, on which it becomes impossible to detect a target.        They reject only signals whose signal to noise ratio is positive after angular compression. This rejection is limiting if it is effected at the beginning of radar processing, i.e. before distance-Doppler compression.        
The invention is more particularly directed to the rejection of all zero Doppler effect signals in the clutter picked up by a passive radar receiver of particular orthogonal frequency division multiplex (OFDM) signals. OFDM signals are characterized by simultaneously emitting a large number of orthogonal sub-carriers phase coded with plural phase or amplitude states, i.e. by a spectrum of orthogonal lines, in the Fourier transform sense, over a finite duration T, equidistant at intervals 1/T.
A bistatic radar disclosed in the French patent application FR 2776438 processes coded OFDM (COFDM) digital radio signals in the context of radio and television broadcasts conforming to the European Digital Audio Broadcasting (DAB) and Digital Video Broadcasting (DVB) standards. These signals, which are therefore sent by emitters of opportunity in the case of passive radar receiver applications, ensure optimum use of the spectrum emitted, in a similar manner to white noise, and are resistant to multipath propagation and interference.
According to the above patent application, the radar receiver comprises a plurality of receive antennas for detecting the signals. The radar processing is based on Doppler-distance correlation of the signals received with a emitted signal time reference. The time reference is obtained by decoding the signals recorded conforming to the radio telecommunications operations effected.
However, because of the bistatic nature of the radar system, the power of the direct path signal is high compared to that of the wanted signal reflected by a target. The direct path should be rejected before effecting the Doppler-distance correlation. The energy contained in the distance-Doppler secondary lobes of the direct path is generally significantly higher than the thermal noise, so that targets situated in the vicinity of the direct path are difficult to detect.
PCT International Application PCT/FR02/00224 filed on Jan. 18, 2002 and not yet published, proposes a radar receiver for OFDM radio signals received via a propagation channel and comprising frames of symbols each emitted on orthogonal coded carriers. The radar receiver seeks to eliminate the contribution correlation of the direct path and more generally of unwanted zero Doppler effect signals to the processing of received signals before Doppler-distance. The radar receiver referred to comprises shaping means for converting the received signal into the form of a digital symbol signal, Doppler-distance correlation means for discriminating mobile targets, and filtering means for eliminating in the symbol signal at least unwanted zero Doppler effect signals in order to apply a filtered signal including essentially signals backscattered by targets to the correlation means. The radar receiver can comprise a plurality of receive channels.
However, although the filtering means based on inverse matrices of covariance matrices each depending on products of spectral lines of the symbol signals relating to a respective carrier reject all of the clutter, i.e. essentially the unwanted zero Doppler effect signals, the receiver does not provide isotropic spatial coverage. After filtering, the radiation diagrams of the filtered signals corresponding to the receive channels feature blind sectors, i.e. “gaps” in the directions in which correlated zero Doppler effect signals are received, especially if the latter signals are at high powers.
Adaptive filtering of the lines of the received OFDM signals causes rejection losses that can result from correlation between the directional vector related to a mobile target and one of the vectors associated with the zero Doppler effect signals that must be filtered. Mobile targets in the blind direction in which the unwanted zero Doppler effect signals are eliminated can no longer be detected.